While processes are known for laminating a film to a film by thermocompression bonding, the dry lamination process is most generally used in which films are bonded together by thermocompression with application of an adhesive. However, this process has problems since an organic solvent is used for the application of the adhesive. The solvent residue poses a hygienic problem, the drying step needed increases the number of steps, the need to vaporize the solvent for drying entails an increased cost, and the air pollution due to the resulting gas is not negligible. With the extrusion lamination process which is practiced as an alternative to the above process, a molten resin film is extruded onto a film directly or with an anchor coat layer interposed therebetween to form a laminate.
The latter process nevertheless has drawbacks. The apparatus is large-sized and costly, variations in the quality of the product present difficulties in quality control, and the extrusion temperature, which is generally high, permits release of odor due to oxidation and is liable to result in impaired heat sealability. Moreover, when the apparatus is started up for producing a small amount of laminate, a very great loss occurs.
In view of the above situation, we have already succeeded in developing a laminate film producing process which uses no solvent, is highly amenable to small-quantity production, can be practiced by a small apparatus at a low equipment cost and affords a product of uniform quality as proposed in Unexamined Japanese Patent Publications Nos. SHO 56-77116, SHO 56-109726, SHO 57-47624, etc. Whereas the conventional dry lamination process employs a liquid or semi-solid adhesive of low viscosity, therefore uses relatively low temperature and pressure for thermocompression bonding and can be practiced by a usual dry laminator free of problems, the process of Unexamined Japanese Patent Publication No. SHO 56-77116, etc., wherein solid films are bonded into a laminate by thermocompression, requires relatively high temperature and high pressure and therefore encounters problems when practiced by the conventional dry laminator for use with an adhesive.
Stated more specifically, problems arise if a laminate film is produced using the conventional dry laminator of the adhesive application type wherein members are fixedly provided in front and rear of its thermocompression unit. For example, when the film feed angle, the laminate film discharge angle, the temperature of the heat roll and the pressure for thermocompression, etc. are set to values which are optimum for a high production speed which is the usual speed of lamination, the laminate film obtained will be creased or clouded owing to excessive heating when the apparatus is at a low speed immediately after start-up or immediately before stopping. Conversely, if these values are so set as to be optimum when the apparatus operates at a low speed immediately after start-up or immediately before stopping, the amount of heat given to the films will be insufficient during a high-speed operation, failing to impart sufficient bond strength to the resulting laminate film. Thus, the laminator is unable to operate satisfactorily in accordance with variations in the production speed.
The object of the present invention is to overcome the foregoing drawbacks of the prior art and to provide a process for laminating films into a laminate free of creasing and clouding and having uniform and sufficient strength, and an apparatus for practicing the process.